Selection bias
MCAT glossary · Psychological, Social & Biological Foundations of Behavior
Definition
Systematic difference between sample and population, biasing results (volunteer bias, healthy-user bias, attrition bias).
Selection bias in plain English
Selection bias happens when the group you actually study isn't a fair mirror of the group you want to learn about, so your results get skewed before you even measure anything. For example, if you test a new workout by studying only people who volunteered for a gym program, they're probably already healthier and more motivated than average — so your 'it works great' finding is really about who signed up, not the workout. Common flavors: volunteer bias (eager people differ), healthy-user bias (health-conscious people do lots of good things at once), and attrition bias (the people who drop out mid-study are often the ones it didn't work for). The lesson: who ends up in your sample can secretly decide your answer, so a great result can be an illusion created by the wrong crowd.
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